An ultrasonic sensor has been proposed that includes multiple sensor devices arranged in an array. For example, such an ultrasonic sensor detects a position (i.e., distance and direction) of an object by calculating a time interval between transmitting an ultrasonic wave and receiving the ultrasonic wave (i.e., echo) reflected from the object and by calculating a phase difference between the ultrasonic waves received by receiving devices.
In an ultrasonic sensor disclosed in JP-A-2007-170975, a synchronous detection of a received signal is performed on each receiving element to detect a phase difference between ultrasonic waves received by the receiving devices. In such an approach, the Doppler effect can be reduced. However, since the synchronous detection is performed on each receiving element, the ultrasonic sensor needs many processing devices. Accordingly, the ultrasonic sensor is increased in size.
In an ultrasonic sensor, a detection error may be caused by a direct wave that is directly received by a receiving device without being reflected by the object. JP-A-2007-170975 discloses a technique for reducing the detection error caused by the direct wave. According to the technique, an electrical signal having an opposite phase to an ultrasonic wave transmitted by a transmitting device is supplied to a detection circuit, which detects an object based on the ultrasonic wave received by a receiving device. The technique may be effective only when the transmitting device and the receiving device are physically separated from each other.
In an ultrasonic sensor disclosed in JP-A-2000-253496, multiple sensor devices are arranged in an array such that a transmitting device and a receiving device are physically joined together through an adhesive. A detection sensitivity of the ultrasonic sensor can be improved by causing the transmitting device to vibrate strongly so as to increase pressure of the ultrasonic wave transmitted by the transmitting device. However, when the transmitting device vibrates strongly to transmit the ultrasonic wave of high pressure, the vibration of the transmitting device is likely to propagate to the receiving device through the adhesive. The vibration propagation from the transmitting device to the receiving device may cause a detection error. The technique disclosed in JP-A-2007-170975 is not effective for reducing the detection error caused by the vibration propagation.